Question: Consider two solid spherical balls, one centered at $\left( 0, 0, \frac{21}{2} \right),$ with radius 6, and the other centered at $(0,0,1)$ with radius $\frac{9}{2}.$  How many points $(x,y,z)$ with only integer coefficients are there in the intersection of the balls?
Solution: If $(x,y,z)$ lies inside the first sphere, then
\[x^2 + y^2 + \left( z - \frac{21}{2} \right)^2 \le 36,\]and if $(x,y,z)$ lies inside the second sphere, then
\[x^2 + y^2 + (z - 1)^2 \le \frac{81}{4}.\]Thus, we are looking for the number of lattice points that satisfy both inequalities.

From the first inequality, $z - \frac{21}{2} \ge -6,$ so $z \ge \frac{9}{2}.$  From the second inequality, $z - 1 \le \frac{9}{2},$ so $z \le \frac{11}{2}.$  Since $z$ must be an integer, $z = 5.$  Then
\[x^2 + y^2 \le 36 - \left( 5 - \frac{21}{2} \right)^2 = \frac{23}{4}\]and
\[x^2 + y^2 \le \frac{81}{4} - (5 - 1)^2 = \frac{17}{4}.\]Since $x$ and $y$ are integers, $x^2 + y^2 \le 4.$

The possible pairs $(x,y)$ are then $(-2,0),$ $(-1,-1),$ $(-1,0),$ $(-1,1),$ $(0,-2),$ $(0,-1),$ $(0,0),$ $(0,1),$ $(0,2),$ $(1,-1),$ $(1,0),$ $(1,1),$ and $(2,0),$ giving us a total of $\boxed{13}$ points.